1. Field of the Invention
The present invention relates to a switching power supply unit that switches an input voltage to generate one or plural output voltages having different values.
2. Description of the Related Art
Portable telephones, digital cameras, PDAs, and personal computers require a plurality of DC voltages having different voltage values as power supply voltages for LCDs (liquid crystal displays) and CCDs (charge-coupled devices).
A transformer is normally used in a power supply unit for obtaining a plurality of DC voltages. A DC power supply unit has a configuration in which a DC input voltage applied to a primary winding of such a transformer is switched by a PWM control signal and a voltage induced at a secondary winding is rectified and smoothed to output a plurality of DC output voltages.
The power supply unit utilizing a transformer has low converting efficiency (e.g., about 50 to 70%) as a power supply unit and is large-sized because the secondary winding has a great number of turns and hence a small diameter in order to achieve strong magnetic coupling between the primary winding and the secondary winding.
Power supply circuits have been proposed which employ two inductors or an inductor having an intermediate tap to obtain a plurality of output voltages (see Patent Document 1, for example). However, the power supply circuits are complicated in circuitry because a multistage charge pump circuit is provided to obtain one output voltage. Loss in the charge pump circuit reduces efficiency, and the circuit requires an extra space which results in an increased size. Referring to the characteristics of each output voltage relative to the input voltage (line regulation), an output voltage that is not feedback-controlled has input voltage dependence.
JP-A-10-150767 is known as a related reference.
Since the switching power supply units according to the related art are low in conversion efficiency and are large-sized as thus described, they have problems when used as power supply units of portable apparatus which must be compact and lightweight and whose battery power supply must have a long life. They have another problem in that each output voltage cannot be kept at a predetermined value at the time of a voltage drop of the battery power supply because it has input voltage dependence.